This invention relates to a method for studying the flow of fluids in subterranean formations and more particularly to tracing the flow of a plurality of injected fluid slugs by use of a number of tracers smaller than the number of injected fluid slugs.
Petroleum is often removed from subterranean formations by injecting water in one or more wells and producing oil and water at one or more other wells. To most efficiently operate such a waterflood production process it is desirable to know the flow pattern of water through the subterranean formations from the injection wells to the production wells. The use of particular materials to trace such water flow is well documented by the disclosures of previous patents.
U.S. Pat. No. 3,508,875, issued to B. B. Sandiford on Apr. 28, 1970, discloses the use of water-soluble iodide, thiocyanate, and salicylate compounds in up to three injection wells at one time. While the use of these three materials was previously known, this patent added the improvement of a simple test procedure which would detect the presence of any one or all of the compounds.
U.S. Pat. No. 3,508,876, issued to O. L. Polly on Apr. 28, 1970, discloses the use of various water-soluble aldehydes as water flow tracers. U.S. Pat. No. 3,851,171, issued to B. B. Saniford, et al., on Nov. 26, 1974, discloses yet another and more complicated chemical compound, water-soluble substituted poly(hydroxyalkyl) bis(triazinylamino) stilbene, which is useful as a water tracer.
As is recognized in the above-referenced patents, the number of practical water tracing compounds is limited by several requirements. Tracers must, of course, be highly soluble in water or other fluid being traced. Useful tracers should be essentially insoluble in formation oil which is typically present. The tracer should not adsorb onto rock surfaces in the formation as the fluid passes through the formation. If the tracer is either soluble in oil or tends to adsorb on the rock, it may be totally lost in the formation or its effective movement through the formation may be slower than the fluid that is being traced.
The fact that certain tracers are delayed by formation oil or rocks is illustrated by U.S. Pat. No. 3,590,923, issued to C. E. Cooke, Jr., on July 6, 1971. In this patent two different tracers are mixed and injected with water to pass through a portion of the formation to a production well. One of the two tracers is delayed in transit due to its having a higher solubility the in immobile oil in the formation than the other. In this disclosure the difference in delay time is used as a measure of the oil saturation of the formation. In a similar way, U.S. Pat. No. 3,902,362, issued to Tomich, et al., on Sept. 2, 1975, discloses the use of two tracers having different partition coefficients in a single well to measure the natural fluid drift of the mobile phase present in the formation.
Thus, while it is seen that the use of tracers of trace water flow in subterranean formations is well-known, the number of useful and preferred tracers is small. Therefore, when the number of injection wells in a producing formation is larger than the number of available and suitable tracers, there is no way of accurately and simultaneously tracing the flow of water from each of the injection wells to a production well.
Secondary and tertiary oil recovery methods make the need for more tracers even more apparent. For example, in a micellar fluid injection process there may be four distinct banks or slugs of fluid injected in each injection well. To accurately monitor the progress of a micellar flood, each of these four banks should have its own tracer and, of course, the tracers for each of a number of wells must be different.
While the above discussion covers specifically the area of oil recovery by use of water or water-based fluid injection, similar flooding processes are known in which gases are injected to displace oil. These gases include petroleum gases such as methane and propane and also carbon dioxide. As with water injection, gases are typically injected into a number of wells and oil produced at other wells. Gaseous tracers are known and are used to trace the flow of injected gas from each injection well to the production well just as in waterflooding. The number of such available gaseous tracers is limited and additional tracers are desirable.